The invention concerns a monochromator for a charged particle optics, in particular, for electron microscopy, comprising at least one first deflection element with an electrostatic deflecting field for generating a dispersion in the plane of a selection aperture for selecting the charged particles of the desired energy interval and at least one second deflection element with an electrostatic deflecting field which eliminates the dispersion of the at least one first deflecting field.
Chromatic aberration is one of the main factors that limits the resolution in the charged particle optics due to the resulting broadening of the charged particle beam due to the width of the energy spectrum and the chromatic aberration of the lenses. The monochromators are used to limit this chromatic aberration. In electron microscopy e.g., the energy width of 0.2 eV must not be exceeded with an acceleration voltage of 200 kV, in order to obtain a resolution of less than 1 Angstrom. Electron sources with the smallest full width at half maximum are thermally supported field emission cathodes, having an energy full width at half maximum of still 0.6 to 1.0 eV. Investigations have shown that approximately 30% of the electrons deviate by less than 0.1 eV. For certain applications, such as transmission or scanning electron microscopes, small beam currents are sufficient, such that it is possible to filter out approximately 70% of the electrons in order to realize a sufficiently monochromatic electron source with sufficient beam currents. Monochromators can therefore be used.
EP 0 470 299 B1 discloses a monochromator, in which hemispherical capacitors with inner and outer hemispherical electrodes are used as deflection elements, which are connected to different potentials. The deflection elements are arranged mirror-symmetrically with respect to a center plane that contains a selection aperture for selecting out electrons of different energy. Since the deflection elements are designed as hemispherical capacitors, the charged particles which are deflected in different directions with respect to the optical axis, are repeatedly focused to points, since the charged particles are deflected in a spherical field which equally influences the charged particles of all sections through the optical path, such that the generated intermediate images of the radiation source are point-like. Two of these point focusings are thereby formed upstream and downstream of the selection aperture, and one point focusing is formed in the opening of the selection aperture.
With point focusing of this type, only the charged particles of the same energy meet at one point. Due to the different energies of the charged particles, the points of the charged particles of different energies are joined and form one line, which enables shielding of the charged particles with an excessive high or low energy by means of the selection aperture, and permits passage only of the charged particles of the desired energy interval. This energy interval is then reunited downstream of the selection aperture by the downstream deflection elements. The dispersion of the upstream deflection elements is thus again eliminated.
One disadvantage of a monochromator of this construction is that the interaction of the charged particles increases the closer they are brought together. This Boersch effect counteracts the desire to obtain a high monchromatism. The charged particles deflect each other, which causes deceleration and acceleration with an additional dispersion effect, which results in an increase of the virtual source size. A monochromator is constructed in such a fashion that it eliminates the dispersion of the part upstream of the selection aperture in the part downstream of the selection aperture. It can thereby reverse the displacements caused by its deflecting fields, but not those displacements caused by the interaction of the charged particles, which influences the monchromatism and thereby the ability of focusing the beam, which again impairs the resolution of the optics.
In view of the above and similar monochromators with point focusing (stigmatic intermediate images), a monochromator of the above-mentioned type was proposed in the dissertation “Design of a monochromator for electron sources” by Frank Kahl (internet: elib.tudarmstadt.de/diss/000030), in which the deflection elements are designed in such a fashion that the charged particles of an x and a y-section describe different paths and only astigmatic intermediate images are produced in the form of line focuses. The proposed monochromator has been patented (DE 196 33 496B4).
This dissertation presents the previous monochromators in section 3.1 thereof (pages 25 to 27), i.a. the monochromator by Rose (3.1.3) which corresponds to EP 0 470 299 B1. In 10.2.3 (pages 144 to 148), the stigmatic and astigmatic optical paths are compared to determine that stigmatic intermediate images (point focuses) produce a multiple source enlargement compared to astigmatic intermediate images (line focuses).
In view of the latter, an enlargement of the source area by a factor of 7 was stated as the “worst case”, the system of Rose, however, states as a “worst case” an enlargement of the source area by a factor of 60 (loc. cit. p. 148).
Since all previous monochromators show the disadvantageous Boersch effect of point focuses (stigmatic intermediate images), the beam current had to be limited to 10 nA (loc. cit. p. 28). This is again disadvantageous, since an increase in resolution requires large illumination currents in connection with a small source energy width. In order to overcome this limitation, it was proposed to allow for astigmatic real intermediate images of the source only (loc. cit. 3.2, page 28), i.e. line focuses. Line focus means that the charged particles of a given energy in the intermediate images of the radiation source are not focused into a point but a line. This line is broadened by the different energies due to dispersion, such that a beam is produced in the focal plane, which has a substantially rectangular cross-section. In order to select a desired energy interval with this focus, the charged particles with differing energies must be selected out and only the line focuses of the desired energy interval are allowed to pass through the aperture. The line focuses require a slit aperture oriented in their direction, whose width determines the selected energy interval (FIG. 8).
This monochromator does avoid the strong Boersch effect of stigmatic intermediate images, but the disadvantage of this monochromator is that unevenness or soiling of the aperture slit also effects the charged particle beam, resulting in scattering and intensity modulations in the final image of the optical system which show up as stripes across the image (FIGS. 8a and 8b). The sensitivity in this regard is sufficiently large that this fault cannot be prevented by mechanical precision and prevention of dirt deposit. In particular, for objects with small intensity contrast such as thin crystals, this causes the intensity contrasts of the object to be superimposed by the intensity contrasts of this defect, which produces stripes in the image that often prevent image evaluation.
It is therefore the underlying purpose of the invention to design a monochromator of the above-mentioned type in such a fashion that a high monchromatism can be achieved without intensity contrasts caused by defects of the aperture.